Ashley Johnson-Barr has been missing since Thursday. She was last seen at the Rainbow Park playground in the Northwest Alaska town around 5:30 p.m. Her family reported her missing when she didn't come home.

The girl's cellphone was found "in the street" about half a mile from the park, according to a statement from Kotzebue Police Chief Thomas Milliette.

Alaska State Troopers are now the lead on the investigation, having taken over at the request of the Kotzebue Police Department, said troopers spokesman Jonathon Taylor. Troopers asked the FBI to assist in the investigation, Taylor said.

When asked whether officials have found evidence of foul play, Taylor said, "We are investigating the incident and exploring all possibilities."

"It's still a search-and-rescue operation at this point," Taylor said.

At least 50 volunteers were searching for Johnson-Barr on Monday, Taylor said.

The Alaska Incident Management Team, a statewide emergency response group, has been assisting with the search since Saturday, Taylor said.

A U.S. Coast Guard helicopter conducted a fly-by of coastal areas, and Alaska Wildlife Troopers aircraft have also been used in the search for the girl, Taylor said.